Considerable interest has been shown in recent years in the use of prostaglandin (PG) precursors in medicine.
For various reasons it is not practical to administer naturally-occurring prostaglandins such as PGE 1 and PGE 2 to patients. Consequently, considerable attention has focussed on the use of prostaglandin precursors including linoleic acid, .gamma.-linolenic acid (GLA) and dihomo-.gamma.-linolenic acid (DGLA).
Conversion of these materials in the body is believed to be as shown in the following diagram: ##STR1##
The broad outline of this pathway is well known, but the details of control, inhibition and enhancement are shown as the present inventor believes them to operate. The pathway is now discussed with particular reference to treatment of malignant tumors susceptible to treatment with gamma-linolenic acid or dihomo-gamma-linolenic acid according to the invention. This discussion is given in the belief that it elucidates the invention, but it is not intended that the invention should be limited by what is believed to be the reason for its effectiveness.
A major function of essential fatty acids (EFAs) is to act as precursors for prostaglandins (PGs), 1 series PGs being formed from dihomo-.gamma.-linolenic acid (DGLA) and 2 series PGs from arachidonic acid (AA). DGLA and AA are present in food in only small quantities, and the major EFA in food is linoleic acid which is first converted to .gamma.-linolenic acid (GLA) and to DGLA and AA. The conversion of linoleic acid to GLA is blocked by a high fat and high carbohydrate diet, by ageing and by diabetes. Stores of AA in the body in the form of lipid esters are very large indeed. In contrast only small amounts of DGLA ester are present.
There is evidence to show that in cancers, there is an overproduction of prostaglandins, abnormal calcium levels and a switch from an oxidative mode of metabolism to a glycolytic mode, which the present inventor believes may be due in part to a defect in the synthesis of TXA2. Such a defect may be caused for example by radiation. Moreover it is also believed that in an organism TXA2 binds to chromosomes and thereby increases their resistance to mutagens.
Thus, it is suggested, cancer involves two mechanisms, firstly a reduction in TAX2 levels and secondly an exposure to a mutagenic agent which leads to uncontrolled growth.
Therefore, many of the problems of cancer may be caused by the primary failure of TXA2 synthesis and the secondary excess of prostaglandins of the 2 series and depletion of prostaglandins of the 1 series and essential fatty acids.
Thus the invention, in one aspect, serves to redress the 1-series PG depletion by administering .gamma.-linolenic acid and/or other materials tending to enhance 1-series PG production. In another aspect, desirably combined with the first, it seeks to restore TXA2 production directly.
It has further recently been found that a critical factor in some inflammatory disorders, e.g. in the damage of myelin which occurs in multiple sclerosis, may be the entry of calcium into cells. This may damage mitochondria and activate destructive lysosomal enzymes. Thus, there is now evidence which indicates that the regulation of the immune response and also the control of intracellular calcium may be significant factors in the treatment of various inflammatory disorders.
The present inventor has now found that colchicine is a substance which appears to be able to potentiate the removal of calcium by cells and thus may be able to control intracellular calcium. Colchicine may also inhibit formation of 2 series PG's and enhance formation of 1 series PG's. In a further aspect of the invention, therefore, in conjunction with correction in EFA balance, colchicine is administered to effect such control The relationship of this to EFA metabolism is discussed later.
In cancers as discussed above, and in inflammatory disorders, production of 2 series PGs from arachidonic acid is greatly exaggerated. In inflammatory disorders these PGs are thought to contribute to the causation of the disease because steroids and aspirin-like drugs are both partially effective therapies, steroids blocking the conversion of AA esters to free AA and aspirin-like drugs blocking the conversion of free AA to endoperoxides which are intermediates in PG synthesis. As yet there is less evidence that the increased formation of 2 series PGs plays an important part in cancer but some human tumours are known to respond to steroids and growth of some animal tumours is inhibited by aspirin-like drugs.
The overproduction of 2 series PGs implies that normal control of the PG synthetic pathway has been lost. Although control of this pathway is imperfectly understood two factors have been identified.
1. PGE1 is able to inhibit the formation of free AA from AA esters. This leads to the paradoxical fact that a partial EFA deficiency actually leads to increased formation of 2 series PGs, because DGLA stores are so much smaller than those of AA and a partial deficiency of EFAs will therefore lead to DGLA depletion first. This depletion will reduce formation of PGE1, remove the PGE1 control of AA and allow overproduction of 2 series PGs from the large AA stores.
2. An unstable product of AA metabolism, thromboxane A2 (TXA2), also feeds back to inhibit conversion of AA ester to free AA and possibly also of free AA to PG2 endoperoxides. Thus loss of TXA2 will also lead to overproduction of 2 series PGs. TXA2 and PGE1 thus cooperate in the regulation of formation of 2 series PGs and a fault in the formation of either will lead to abnormalities.
Thus for example the disorders of PG synthesis in inflammatory disorders can be accounted for by inadequate formation of PGE1 and/or TXA2.
The evidence for direct involvement of PGs in inflammatory disorders and cancer has been briefly mentioned. There is also indirect evidence that PGs may act by regulating--or failing to regulate--the calcium movements into and out of cells already mentioned above. The calcium concentration in cytoplasm is normally very low and there is now excellent evidence from many sources that a brief rise in cytoplasmic calcium concentration triggers a variety of cell events, including cell division and activation of lysosomes which contain destructive enzymes. Normally this calcium is very rapidly removed after this brief activation so terminating the event. PGs and related substances have specific actions on calcium and the present inventor has obtained evidence to suggest that TXA2 and PGF2.alpha. may be of critical importance. In particular, specific inhibition of TXA2 synthesis greatly prolongs the time taken for calcium to be removed from the cytoplasm after activation. Furthermore, inhibition of TXA2 synthesis leads to increased formation of PGF2.alpha. and PGE2 which can promote calcium entry into cells. There is thus good evidence that in this respect also PGE1 and TXA2 enhance one another's effects. In particular, in muscle the degree of contraction is related to the calcium concentration in the cytoplasm and muscle contraction is a measure of this calcium concentration. After inhibition of TXA2 synthesis the recovery from a contraction is greatly prolonged indicating slow removal of calcium. Further, inhibition of TXA2 synthesis can lead to a chronic state of partial contraction indicating the entry of calcium into the cytoplasm. PGF2.alpha. and PGE2 whose output is increased by inhibition of TXA2 synthesis also cause contraction indicating calcium entry into the cytoplasm.
Thus loss of TXA2 and PGE1 synthesis will lead to increased formation of 2 series PGs and entry of calcium into the cytoplasm. This calcium may activate cell division and also activate lysosomes whose destructive enzymes may play a large part in inflammation.
There is a good deal of evidence that cancers do indeed not produce TXA2 normally. The most striking is as follows:
(a) Specific inhibitors of TXA2 synthesis, such as imidazole, can produce in normal cells biochemical abnormalities similar to those in naturally occurring cancers.
(b) Radiation and phorbol esters, which powerfully promote the development of cancers, are both able to inhibit the enzyme which forms TXA2.
The evidence of defective 1 series PG synthesis in cancer is less substantial at present. However, rapidly growing cancers frequently produce skin lesions in their hosts which are identical to those caused by 1 series PG deficiency. Further, in rat breast cancer there is evidence that synthesis of .alpha.-lactalbumin is regulated by PGE1, and .alpha.-lactalbumin synthesis fails as the breast tissue is transformed from the normal to the cancerous state.
There is suggestive evidence that TXA2 may be able to protect DNA from mutations. For example the phorbol esters do not cause mutations themselves but they do make cells much more susceptible to other mutagenic agents, or more particularly, the expression of their effect. It is possible that even when a mutation has taken place, it may not be expressed if adequate amounts of TXA2 are present. For example rats can be exposed to mutagenic radiation at birth but develop cancers only on administration of phorbol esters up to a year later.
On general ground there are therefore reasons to suppose that suppression of excess production of 2 series PGs will have desirable effects in both inflammatory disorders and cancer. Currently available conventional methods of suppression are administration of steroids and aspirin-like drugs. However, while these may suppress overproduction of 2 series PGs they will exaggerate further any deficiencies in PGs of the 1 series and in TXA2, which may explain why they control symptoms and do not usually alter the long term course of the disease.
The present invention proposes a radically new approach which will control excess PG2 series production by restoring towards normal, or enhancing, the formation of either or both of 1 series PGs and TXA2.
The methods proposed for doing this are as follows:
1 series PGs
To increase the available supply of precursors of 1 series PGs by providing adequate amounts of GLA or DGLA which will bypass any metabolic block between LA and GLA. The GLA or DGLA may be either synthetic or found in natural products. The formation of 1 series PGs may be enhanced further by the adminstration of pharmacological agents with the GLA or DGLA. Agents which have this effect are listed later in the specification. They include penicillamine and levamisole which have both been used as anti-inflammatory agents in rheumatoid arthritis with a completely unknown mechanism of action.
TXA2
To enhance the formation of TXA2 by means of agents which specifically activate the enzyme which forms TXA2 from PG2 series endoperoxides. These agents also are listed later in the specification, and include colchicine and related compounds such as the Vinca alkaloids.
These latter should be used in much lower doses than those at present used in cancer therapy, since high doses may have the reverse effect of inhibiting TXA2 formation. One has the apparently paradoxical situation that colchicine and the Vinca alkaloids may attack cancer in one of two ways. Low doses, according to the invention, activate TXA2 synthesis, inhibit formation of other 2 series PGs and restore calcium regulation. They will therefore tend to normalise cancer cells. High doses on the other hand, as given in known treatments, seem to be toxic to the enzyme. They therefore eliminate any remaining TXA2 synthesis, further enhance formation of PGF2.alpha. and other 2 series PGs and kill the cells by increasing calcium entry to the toxic level the effect nevertheless being sufficiently selective for cancer cells, to be of value.
It may be remarked that radiation also has apparently paradoxical effects which are explained in the concept on which the invention is based. Sub-lethal irradiation of normal cells inactivates TXA2 synthesis, opening the way to the abnormalities seen in cancer. Irradiation of cells in which TXA2 synthesis is already defective kills the cells by overloading them with calcium, the effect therefore being selective to cancer cells.
Direct evidence of effectiveness in the treatment of malignant tumors susceptible to GLA or DGLA is given at the end of the specification.